Aircraft control surfaces, for example flaps located on the trailing edge of a fixed wing, may be driven by rotary actuators, sometimes referred to as “geared rotary actuators” or “GRAs”, as part of a high-lift system of the aircraft. A drive strut may be arranged to transmit loads between an output crank of the GRA and the flap. More specifically, the drive strut may have a first end rotatably coupled to the GRA output crank and a second end rotatably coupled to the flap, wherein rotational motion of the GRA crank is transmitted to the flap to cause the flap to change position in a manner determined by a mounting linkage associated with the flap. The drive strut also acts in arresting and holding the flap in any gated or intermediate position of the flap. The GRAs in such high-lift system may be responsive to motion commands from a slat flap control computer (“SFCC”).
In some aircraft flap systems, the drive strut is configured as a load sensing drive strut (“LSDS”) capable of measuring loading conditions experienced by the LSDS and providing a load signal to the SFCC or to another control device indicative of loading experienced by the LSDS.
Some strut applications, including the LSDS aircraft application described above, require a very high load carrying capability for normal usage but also must sense when a structural disconnect occurs. If the drive strut becomes disconnected from either the GRA or the flap, the high-lift system will malfunction. In the event of a disconnect malfunction, it is desirable to prevent further flap movement commands from being sent to the GRA to mitigate structural damage. In the present example of an aircraft flap system, the ratio of the loading region of interest where a structural disconnect may occur to the ultimate load carrying capability of the drive strut may be a factor of 40 to 50. Furthermore, the requirement for accuracy in the loading measurement, on a full scale basis, may be 1 part in 400 (0.25%) or even tighter. This presents a challenge in the design of a LSDS.